1. Field of the Invention
The present invention relates to a receiving device and an automatic gain control method.
2. Related Art
In a conventional receiver, in order to keep the level of an input signal to an A/D converter constant, the level of the input signal is adjusted by using a variable gain amplifier. The gain of the variable gain amplifier is controlled depending on the level of a received signal and the level of an output signal from the A/D converter. This control is called automatic gain control (AGC).
As a method of automatic gain control, a method obtained by combining a feedforward control method and a feedback control method is known (for example Japanese Patent Publication Laid-open No. 10-56343 (p. 3 and FIG. 1) to be referred to as Patent Document 1 hereinafter).
In the automatic gain control method described in Patent Document 1, the RSSI (Received Signal Strength Indicator) of an analog received signal which has not been input to a variable gain amplifier is detected by a RSSI detector, and AGC coarse adjustment for controlling the gain of the variable gain amplifier is performed on the basis of the RSSI (feedforward control). After the AGC coarse adjustment is performed, a power of a digital signal converted by the A/D converter is measured by a power measurement unit. On the basis of the measured power, AGC fine adjustment for controlling the gain of the variable gain amplifier is performed (feedback control).
In this manner, according to the automatic gain control method described in Patent Document 1, the automatic gain control is performed in two steps of the AGC coarse adjustment and the AGC fine adjustment. For this reason, control time can be made shorter than that in an automatic gain control method which feedback control is performed a plurality of times on the basis of the power of a digital signal serving as a result of A/D conversion.
As described above, in the invention described in Patent Document 1, although automatic gain control can be increased in speed, a RSSI detector must be additionally arranged to perform AGC coarse adjustment. For this reason, a circuit scale increases and a power consumption disadvantageously increase. In particular, since a mobile terminal or the like requires a small size and a low power consumption, the problem of the circuit scale is serious for the mobile terminal or the like.